The present invention relates to an ink detector for a liquid-ink printing element, and more particularly to an ink detector for indicating when a minimum amount of ink remains in the reservoir which feeds an ink-jet print head.
In thermal ink-jet heads, which are well known in the art, the nozzles, the spray chambers and the heating elements are formed on a multi-layer silicon-based chip manufactured by the known processes of vacuum deposition employed in the manufacture of integrated circuits.
In the case of multiple-nozzle heads, the various spray chambers are connected in parallel to a common collecting channel. This is connected in turn through a feed duct to a reservoir or replaceable ink cartridge.
An example of a multiple-nozzle thermal ink-jet print head is described in U.S. Pat. No. 4,812,859, in which a multi-layer silicon-based chip containing the spray chambers, the heating elements and the nozzles is attached directly to the ink reservoir. The reservoir contains a sponge impregnated with ink and is mounted on the carriage of a printer.
The use of sponges to reduce and stabilise the hydrostatic head in the outlet duct of an ink reservoir is also known from U.S. Pat. No. 4,630,758, in which it is shown that the sponge causes a pressure drop in the reservoir outlet duct by capillary action.
In European Patent Application published under No. 408,241 on 16 Jan. 1991, a thermal ink-jet print head is described in which the ink reservoir is of the replaceable cartridge type and is inserted on a support to which is fixed the multi-layer silicon-based chip containing the spray chambers, the heating elements and the nozzles. The cartridge contains an ink-soaked sponge and is able to form a fluid connection with the support to supply ink to the multi-layer chip.
However, the patents cited above make no mention of possible ways of monitoring the amount of ink remaining in the cartridge.
A number of devices for monitoring the amount of ink contained in a reservoir feeding a print head are known from U.S. Pat. Nos. 4,183,029, 4,196,625 and 4,202,267.
These patents relate in particular to feed reservoirs that are completely filled with ink and do not use a sponge. Electrodes are arranged in pairs on the floor of the reservoir next to the outlet duct, or, as in U.S. Pat. No. 4,183,029, directly inside the feed duct between the reservoir and the print head.
Measuring circuits connected to the electrodes measure the variation in the resistance of the ink between each pair of electrodes, and this corresponds to the decrease in the amount of ink remaining in the reservoir. When the electrodes become uncovered by the ink, the measuring circuit indicates that the ink has run out.
In these devices, the resistance of the ink varies slowly as the ink in the reservoir is used, and suddenly climbs to a very high level when the ink is finished.
It is therefore very difficult to determine when one has arrived at a minimum amount of ink before it is exhausted, so as to replace the cartridge or reservoir.
What is more, these devices cannot be adapted to reservoirs and/or cartridges containing an ink-soaked sponge, even if high-quality sponges are used such as those with a three-dimensional cross-linked structure of melamine-formaldehyde resin, as described for example in U.S. Pat. No. 4,929,969.
In this case, the balance between the capillarity of the nozzles and the capillarity of the terminal area of the sponge, close to the floor of the container, causes print deterioration or stoppage before the sensors on the floor of the container or in the outlet duct detect a significant rise in the increase of the resistance of the ink.
There is also known, from European Patent Application published under No. 370,765, another kind of end-of-ink detector formed by two electrodes placed in the ink feed duct leading to the spray chambers of a thermal ink-jet print head. One electrode is sited close to the spray chamber, while the other electrode is arranged upstream, towards the reservoir. A detector circuit indicates variations in the resistance of the ink between the two electrodes.
This type of detector, too, when applied to a reservoir containing an ink-soaked sponge, has the same disadvantages as indicated above.